Tougheners are agents used in curable compositions for giving an improved toughness and, thus, a higher dynamic strength to the cured product. Tougheners are particularly useful in structural adhesives. Structural adhesives are adhesives used for the assembly of the parts of a vehicle such as a car, a lorry, a bus or a train. After curing, structural adhesives have to bear both high static and high dynamic loads. Conventional structural adhesives are epoxy compositions. Cured epoxy compositions per se have a relatively high static strength (i.e. a high tensile and a high lap shear strength), but a rather poor dynamic strength (i.e. a low impact peel strength). In order to fulfill the requirements of a crash resistant structural adhesive having a high dynamic strength, a toughener is usually added to the epoxy composition.
Several epoxy adhesive compositions have been described in the state of the art. U.S. Pat. No. 6,015,865 relates to an adhesive composition comprising a liquid and a solid epoxy resin and an amino terminated polyalkylene glycol. JP-A-02150485 describes an adhesive composed of a gelatinized epoxy resin, a latent curing agent and an electrically conductive material. A portion of the epoxy resin composition comprises a urethane bond and is capped with the polyhydric derivative of Bisphenol A diglycidylether. JP-A-02199116 discloses an epoxy resin composition comprising an epoxy compound having a urethane bond and a curing agent. JP-A-05156227 discloses a structural adhesive obtained by formulating a urethane-modified epoxy resin, an acrylic rubber-modified epoxy resin and/or a polyalkylene ether-modified epoxy resin with a potential curing agent. JP-B-02749610 discloses a urethane modified epoxy resin giving flexibility to a cured product. JP-B-07042449 describes a structural adhesive comprising an epoxy resin, a curing agent, a conductive ingredient and a plasticizer. WO-A-00/20483 relates to epoxy adhesive compositions comprising a conventional rubber-modified epoxy resin and a condensation product of a carboxylic acid dianhydride with a di- or polyamine and a polyphenol or aminophenol. WO-A-01/94492 describes the condensation products of cyclic carboxylic acid anhydrides and difunctional polyamines and their use as structural constituents for epoxy adhesive compositions. WO-A-03/078163 discloses a curable adhesive comprising an epoxy based prepolymer, an acrylate terminated urethane resin and a heat-activated curing agent. EP-A-1 431 325 describes an epoxy adhesive composition comprising an epoxy adduct, a polymer of a defined formula, a thixotropic agent and a hardener. EP-A-0 307 666 discloses phenol-terminated polyurethane or polyurea compounds and epoxy resins comprising these compounds. WO-A-05/007720 relates to thermohardening compositions comprising low-temperature impact strength modifiers. U.S. Pat. No. 6,776,869 describes an adhesive comprising a polymer having at least one glass transition point of −30° C. or lower, a reaction product of a polyurethane prepolymer and a polyphenol or polyaminophenol, and an epoxy resin.
The mechanical properties of the above described epoxy adhesive compositions do generally not fully comply with the requirements of a structural adhesive having both a high static and dynamic strength.
EP-A-0 308 664 discloses an epoxy adhesive composition comprising a butadiene-acrylonitrile copolymer in combination with a polyphenol-terminated polyurethane or polyurea as a toughener. The tougheners described in EP-A-0 308 664 are known to give a good degree of toughness of the cured epoxy composition in which it is comprised. However, it does not teach how a further improvement in toughness is achievable. These tougheners are however expensive due to the limited availability of the aminophenol and polyphenol used for preparing the tougheners.
It is therefore an object of the present invention to provide a toughener for a heat-curable, storage-stable structural epoxy adhesive, said toughener being based on readily available starting materials and having excellent toughening properties.